Bearing steels

ABSTRACT

There is provided a novel bearing steel composition and a method of forming a bearing. The bearing steel composition comprises: Carbon 0.4 to 0.8 wt %; Nitrogen 0.1 to 0.2 wt %; Chromium 12 to 18 wt %; Molybdenum 0.7 to 1.3 wt %; Silicon 0.3 to 1 wt %; Manganese 0.2 to 0.8 wt %; and Iron 78 to 86.3 wt %.

TECHNICAL FIELD

The present invention relates to the field of steels and bearings. Morespecifically, the present invention relates to a novel bearing steelcomposition and a method of forming a bearing.

BACKGROUND

Bearings are devices that permit constrained relative motion between twoparts. Rolling element bearings comprise inner and outer raceways and aplurality of rolling elements (balls or rollers) disposed therebetween.For long-term reliability and performance it is important that thevarious elements have a high resistance to rolling contact fatigue, wearand creep.

Modern steels are made with varying combinations of iron and alloyingelements depending on the desired application of the steel product.Chromium is a common alloying element and is used to resist corrosionand rusting. Stainless steels and surgical stainless steels contain aminimum of 10% chromium.

An example of a stainless steel composition is Böhler N360 Isoextra®.This steel comprises 0.30 wt % carbon (C), 0.60 wt % silicon (Si), 0.40wt % manganese (Mn), 15.00 wt % chromium (Cr), 1.00 wt % molybdenum (Mo)and 0.40 wt % nitrogen (N). The steel is a corrosion resistant,martensitically hardened stainless steel which exhibits high hardnessand compressive strength.

The Böhler N360 Isoextra® steel is produced using a pressurisedelectroslag remelting (P-ESR) process. In the P-ESR process a consumableelectrode is dipped into a pool of slag in a water-cooled mould under aprotective atmosphere. An electric current (usually AC) passes throughthe slag, between the electrode and the ingot being formed andsuperheats the slag so that drops of metal are melted from theelectrode. The drops travel through the slag to the bottom of thewater-cooled mould where they solidify. The slag pool is carried upwardsas the ingot forms. The new ingot of refined material builds up slowlyfrom the bottom of the mould. The ingot formed is homogeneous andovercomes problems associated with core weakness in conventionally castingots as they solidify from the outside inwards.

The protective atmosphere used to produce Böhler N360 Isoextra®comprises nitrogen to increase the nitrogen content of the steelproduced. Solid nitrogen-bearing additives are also added to the melt.Nitrogen is attractive as an inexpensive alloying element for enhancingthe properties of steel. Due to the extremely short dwell time of themetal droplets in the liquid phase during remelting, the nitrogenpick-up via the gas phase is minimal but the high pressure in the systemprevents the escape of nitrogen from additives introduced into themolten steel.

Various defects, such as the formation of tree ring patterns andfreckles, can occur in the remelted ingots. Furthermore, dendriteskeletons or small broken pieces from the electrode may producestructural defects in the ingot. The process cannot be used onsegregation-sensitive alloys which can be undesirably stirred whilstmolten by stray magnetic fields. In order to avoid the effects of straymagnetic fields the furnaces are commonly designed to be coaxial andthis limits the size and shape of the steel that can be produced.

It is an objective of the present invention to provide an alternativesteel composition, to provide an alternative method of producing thecomposition, and to overcome or at least mitigate some of the problemsassociated with prior art.

SUMMARY

According to a first aspect, the present invention provides a bearingsteel composition comprising:

Carbon about 0.4 to about 0.8 wt %; Nitrogen about 0.1 to about 0.2 wt%; Chromium about 12 to about 18 wt %; Molybdenum about 0.7 to about 1.3wt %; Silicon about 0.3 to about 1 wt %; Manganese about 0.2 to about0.8 wt %; and Iron about 78 to about 86.3 wt %.

Preferably the iron forms the balance of the composition, together withunavoidable impurities.

According to a second aspect, the present invention provides a bearingcomponent formed from a composition as herein described.

According to a third aspect, the present invention provides a bearingcomprising a bearing component according to the present invention.

According to a fourth aspect, the present invention provides a method offorming a steel bearing component. The method comprises the steps ofproviding a powdered steel composition according to the presentinvention and subjecting it to hot isostatic pressing to form thecomponent.

The present invention will now be described further with reference tothe accompanying drawings, provided by way of example, in which:

FIG. 1 shows a schematic diagram of an embodiment of the method of thepresent invention.

FIG. 2 shows a flowchart of the steps taken in an embodiment of themethod of the present invention.

FIG. 3 shows a phase diagram of the composition of the present inventionwith changing carbon content and with a constant 0.15 wt % nitrogencontent.

The present invention will now be described further. In the followingpassages different aspects/embodiments of the invention are defined inmore detail. Each aspect/embodiment so defined may be combined with anyother aspect/embodiment or aspects/embodiments unless clearly indicatedto the contrary. In particular, any feature indicated as being preferredor advantageous may be combined with any other feature or featuresindicated as being preferred or advantageous.

The bearing steel composition includes nitrogen in an amount of from 0.1to 0.2 wt %, preferably from 0.11 to 0.18 wt % and more preferably from0.13 to 0.17 wt %. The nitrogen content serves to increase the hardnessof the steel article. However, the nitrogen concentration in austeniteduring steel processing at high temperature is dependent on thechromium, manganese and molybdenum concentrations and, accordingly, itis difficult to introduce more nitrogen into the composition unless thecontent of these alloying elements is increased. Additionally, resortingto pressure metallurgy can increase nitrogen solubility.

Carbon is included in an amount of from 0.4 to 0.8 wt %, preferably from0.45 to 0.7 wt % and more preferably from 0.45 to 0.6 wt %. The carbonserves to increase the hardness of the steel article formed. If there isless than 0.4% carbon then the steel will be insufficiently hard. Thecarbon solubility is inversely related to the chromium concentration; ifthe chromium weight percent is increased then the carbon content in thefinal composition is likely to decrease.

Preferably the combined percentage of carbon and nitrogen present isfrom 0.5 to 1 wt %, more preferably from 0.5 to 0.88 wt %, morepreferably from 0.5 to 0.77 wt % and most preferably from 0.5 to 0.7 wt%. These ranges have been found to provide the greatest hardness of thefinal steel composition.

In order to compensate for the difficulties in introducing more nitrogeninto the composition, without resorting to the complex further steps ofpressure metallurgy, the amount of carbon is preferably from 0.45 to 0.6wt %. This helps provide the preferable total amount of carbon andnitrogen as noted above. This amount of carbon content with a nitrogencontent of approximately 0.15 wt % allows a maximum amount of solubleinterstitial atoms to be retained in the austenite, or martensite afterhardening, to provide acceptable hardness/load carrying properties.

FIG. 3 shows a calculated pseudo-binary (isopleth) diagram for thecomposition of the present invention having a nitrogen content of 0.15wt % and a variable carbon content. At a carbon content of 0.3 wt % theδ-ferrite phase forms at about 1227° C. The austenite region appears tobe at its widest (in terms of temperature) with a carbon content ofabout 0.45 wt %, which allows for best workability.

Assuming full carbon solubility, for a 0.45 wt % carbon and 0.15 wt % orgreater nitrogen austenite composition, the total C+N content wouldamount to about 0.6 wt %. This value would provide a hardness values ofgreater than 58 HRC. FIG. 3 shows that the carbon content may beincreased further as the γ/γ+L (liquid) phase line is not steeplydecreasing.

Chromium is included in an amount of from 12 to 18 wt %, preferably from15 to 17 wt % and more preferably from 15.5 to 16.5 wt %. The chromiumprovides an improved corrosion resistance property to the steel. Thechromium leads to a hard oxide on the metal surface to inhibitcorrosion.

Molybdenum is included in an amount of from 0.7 to 1.3 wt %, preferablyfrom 0.9 to 1.1 wt %. Adding molybdenum to the steel imparts toughnessfor heavy service, and provides especially heat-resistant alloys.Conventional carbon steels have less than 0.5 wt % Mo, but having agreater amount of molybdenum in the present composition, in conjunctionwith the other alloying elements, improves mechanical properties such ashardness.

Silicon is included in an amount of from 0.3 to 1 wt %, preferably from0.5 to 0.9 wt %.

Manganese is included in an amount of from 0.2 to 0.8 wt %, preferablyfrom 0.3 to 0.6 wt %. The manganese increase hardenability andcontributes to the steel's strength.

Iron is included in an amount of from 78 to 86.3 wt % and preferably asthe balance of the composition, together with unavoidable impurities.

Other elements which may be present include oxygen, phosphorus andsulphur. It is preferred that these elements are present in an amount of0.02 wt % or less. The phosphorous content preferably does not exceed0.01 wt %. The sulphur content preferably does not exceed 0.002 wt %.The oxygen content preferably does not exceed 0.0001 wt %.

It will be appreciated that the steel for use in the bearing componentaccording to the present invention may contain unavoidable impurities,although, in total, these are unlikely to exceed 0.5 wt. % of thecomposition. Preferably, the alloys contain unavoidable impurities in anamount of not more than 0.3 wt. % of the composition, more preferablynot more than 0.1 wt. % of the composition. The phosphorous and sulphurcontents are preferably kept to a minimum.

The alloys according to the present invention may consist essentially ofthe recited elements. It will therefore be appreciated that in additionto those elements which are mandatory other non-specified elements maybe present in the composition provided that the essentialcharacteristics of the composition are not materially affected by theirpresence.

Prior to hardening, the steel according to the present invention willtypically comprise austenite as the predominant phase. That is, at least50 wt %, preferably at least 70 wt % and more preferably at least 90 wt% austenite. In a preferred embodiment, the composition is substantiallyall austenite. That is, at least 95 wt %, more preferably 98 wt % andmost preferably at least 99 wt % austenite. Austenite is a metallic,non-magnetic solid solution of carbon and iron that forms inconventional steels above a critical temperature of 723° C. It has aface-centred cubic (FCC) structure that allows it to hold a highproportion of carbon in solution. When the steel composition issubjected to hardening and tempering it will exhibit a temperedmartensite structure with some retained austenite as well ascarbides/carbonitrides. The phases and structures of steel are wellknown in the art.

If austenite is cooled slowly, then the structure can break down into amixture of ferrite and cementite (usually in the structural formspearlite or bainite). Rapid cooling can result in martensite beingformed. The rate of cooling determines the relative proportions of thesephases and therefore the mechanical properties (e.g. hardness, tensilestrength) of the steel. Quenching (to induce martensitictransformation), followed by tempering (to break down some martensiteand retained austenite, as well as to precipitate somecarbides/carbonitrides), is the most common heat treatment forhigh-performance steels. Deep cooling treatments after hardening and/ortempering stage(s) may also be applied.

For bearing applications the steel composition of the present inventionpreferably has a microstructure comprising martensite, any retainedaustenite and precipitated carbides and/or carbonitrides.

The method employed in the present invention is a form of powdermetallurgy. Powder metallurgy typically relies on a forming andfabrication technique comprising three major processing stages:

-   -   Powdering: the material to be handled is physically powdered and        divided into many small individual particles.    -   Moulding: the powder is injected into a mould or passed through        a die to produce a weakly cohesive structure close in dimension        to the desired product.    -   Compression: the moulded article is subjected to compression and        optionally high temperature to form the final article.

The method of the present invention forms at least a wear portion of abearing component from a bearing steel composition as set out above. Thesteel comprises a number of elements in specified amounts.

The composition used in the method preferably corresponds to thecomposition of the final article produced. However, while the weightpercentage of most of the elements will remain essentially constant, thenitrogen content may decrease slightly, perhaps due to degassing. Whilethe composition initially produced by the hot isostatic pressing step ofthe present invention is predominantly austenitic, the composition ispreferably subjected to further processing steps to introduce amartensite phase. Accordingly, a bearing component made according to thepresent invention will generally comprise a martensitic microstructurehaving some retained austenite (1-30 wt %) and somecarbides/carbonitrides (<5 wt %).

It is preferred that the powdered steel composition is first formed byconventional techniques, for example, by melting suitable ingredients ina melting crucible. The melted components can then be powdered for usein the method of the present invention.

Suitable ingredients include the raw elements or oxides or salts of theingredients which can be decomposed on heating. The ingredients arepreferably melted at a temperature in excess of 1500° C. to ensure thatthe composition is fully molten. If the temperature is lower thenprecipitation of a solid fraction such as δ-ferrite or the like mayoccur. The melt can be produced in an induction furnace and/or tappedinto an induction heated ladle where further alloying elements can beadded in a protective atmosphere. The melt can be stirred andtemperature controlled throughout the process. Prior to the atomisationthe melt can be tapped into a tundish where the melt is protected by,for example, a protective slag cover or an inert atmosphere.

Each of the powder metallurgical steps will now be discussed in furtherdetail in relation to the method of the present invention.

A first step is to provide a powder of the steel composition. A powderis a dry, bulk solid composed of a large number of very fine particlesthat may flow freely when shaken or tilted. Powders are a specialsub-class of granular materials. In particular, powders refer to thosegranular materials that have the finer grain sizes, and that thereforehave a greater tendency to form clumps when flowing. According to thepresent invention, the powder of the steel composition preferably has aparticle size of from 1 μm to 1000 μm, preferably 10 μm to 250 μm andmost preferably 50 μm to 125 μm.

One technique for producing the powder is atomisation. This technique isespecially preferred as it allows the trapping of a particularlypreferred phase from a molten steel composition. Other techniques areknown, such as comminution, grinding, chemical reactions, centrifugaldisintegration, or electrolytic deposition.

Atomization may be accomplished by, for example, forcing a molten metalstream through an orifice at moderate pressures or by disintegration ofa molten metal stream by a high pressure gas stream in an atomisationchamber. As it contacts the gas or a nozzle or an orifice thecomposition may drop in temperature; this can favour austeniteformation. Preferably the chamber is filled with gas to promote furtherturbulence of the molten metal jet. Preferably the atomisation is causedby contacting the molten steel composition with one or more jets of gas,preferably air, nitrogen or an inert gas, for example.

Alternatively, simple atomization may be used by forcing a liquid metalthrough an orifice at a sufficiently high velocity to provide thenecessary turbulent flow to produce powder. Nozzle vibration, nozzleasymmetry, multiple impinging streams, or molten-metal injection intoambient gas can all be used to increase the extent of the atomization.

Once the steel composition has been provided as a powder it canoptionally be stored under inert gas hermetically-sealed vessels.Alternatively, it can be used with minimal delay.

In a second step the powder is placed in a mould. In one embodiment thiscomprises canning the powder in capsules of mild steel, which areproduced by sheet metal forming and welding. Such a mould or capsulewould be designed to give the end product the desired shape.Alternatively, the mould may be formed of heat resistant materials thatare known in the art. Compound products can be produced by designingcapsules or moulds with separate compartments for different powders orenclosing parts of solid material together with the powder.

The mould is preferably subjected to elevated temperature and a veryhigh vacuum to remove air and moisture from the powder. The mould isthen preferably sealed before being hot isostatically pressed. Theapplication of high inert gas pressures and elevated temperaturesresults in the removal of internal voids and creates a strongmetallurgical bond throughout the material. The result is a cleanhomogeneous material with a uniformly fine grain size and a near 100%density.

During hot isostatic pressing (HIP) the filled mould or capsule isplaced in a conventional HIP press where it is subjected to highpressure and temperature. The HIP parameters of pressure, temperatureand time are predetermined in order to give the material full densityand the required properties. Suitable temperatures are those where thecomposition is in the y phase (e.g. the y region in the phase diagram inFIG. 3). That is, from about 1350 to 1100° C., more preferably from 1300to 1200° C. Suitable pressures may be up to 200 MPa but ideally areabout 100 MPa. The pressure may be in the range of from 10 to 150 MPaand most preferably from 95 to 105 MPa. Treatment times from a coldstart (i.e. including the time necessary for the temperature to beramped up) are preferably from 1 minute to 24 hours, more preferablyfrom 1 hour to 10 hours and most preferably from 2 hours to 8 hours.

Preferably the step of hot isostatic pressing is conducted under aninert atmosphere. Suitable inert atmospheres include noble gases such asargon. In another preferred embodiment the hot isostatic pressing isconducted under air to avoid overly complicating the method andapparatus.

According to one embodiment of the present invention the method canfurther comprise a step of nitriding (or case hardening) the hotisostatically pressed bearing component (article). Such processing stepsare intensive and undesirable since the method of the present inventionprovides a suitably hard product without requiring such steps. Thenitriding may serve to harden at least a surface portion of the article.The finished article or composition can therefore be provided with ahigher nitrogen content than could otherwise be achieved.

Suitable nitriding processes are well known in the art and include: gasnitriding; liquid or salt bath nitriding; and ion or plasma nitriding.In gas nitriding the donor is a nitrogen rich gas usually ammonia (NH₃)which is contacted with the pre-heated article. Preferably this isperformed at 200 to 800° C. and is allowed to dwell for at least 30minutes. When ammonia comes into contact with the heated work piece itdisassociates into nitrogen and hydrogen. The nitrogen then diffusesfrom the surface into the core of the material.

In liquid or salt bath nitriding the nitrogen donating medium is anitrogen containing salt such as cyanide salt. The temperature used istypically 550-600° C.

Plasma nitriding relies on a plasma of nitrogen ions and allows forfocused and specific nitriding.

The method of the present invention can further comprise finishing stepsand/or tempering and/or annealing steps. In addition to the deep coolingtreatment mentioned above, another possible step can be stress reliefannealing of a hardened article. Such steps are well known in the art.Depending on the type of material and the application, the products maybe heat treated, machined and subjected to various types of qualitycontrol, such as ultra-sonic inspection, dye penetrant testing, testingof mechanical properties, etc. If a mild steel sheet is used as themould then it can be removed by machining or by acid pickling.

Examples of finishing steps include grinding and polishing. Examples oftempering steps include tempering at between 100 and 500° C., morepreferably 200 to 450° C., for 10 minutes to 24 hours, more preferably30 minutes to three hours. If a hardening step such as nitriding is usedthen it may be desirable to cool the article to a sub-zero temperature(preferably −70° C. to −80° C.) after hardening and before tempering.Preferably the subzero treatment is at −80° C. for 2 hours under air.Preferably tempering, which can be conducted without other hardeningsteps, is performed at 200° C. for two hours under air.

Another finishing step is hot working or hot rolling. These techniquesare well known in the art. Hot working refers to processes where metalsare plastically deformed above their recrystallization temperature.Being above the recrystallization temperature allows the material torecrystallize during deformation. This is important becauserecrystallization keeps the materials from strain hardening, whichultimately keeps the yield strength and hardness low and ductility high.The processing temperature is commonly 0.6 of the materials meltingtemperature. Hot rolling is not especially preferred but may be used toarrive at the final desired dimensions.

Hot rolling is a hot working metalworking process where large pieces ofmetal, such as slabs or billets, are heated above theirrecrystallization temperature and then deformed between rollers to formthinner cross sections. Hot rolling produces thinner cross sections thancold rolling processes with the same number of stages. Hot rolling, dueto recrystallization, will reduce the average grain size of a metalwhile maintaining an equiaxed microstructure.

Using the method of the present invention rather than, for example,P-ESR, the inventors have discovered that it is possible to produce abearing component that exhibits high corrosion resistance. Furthermore,the method of the present invention provides a number of advantages overconventional methods.

The reduced porosity of hot isostatically pressed materials enablesimproved mechanical properties and increased workability. The HIPprocess eliminates internal voids and creates clean, firm bonds andfine, uniform microstructures. These characteristics are not possiblewith welding, casting or P-ESR. The virtual elimination of internalvoids enhances part performance and improves fatigue strength. A furtheradvantage of the HIP process is its ability to create near-net shapesthat require little machining.

In melting operations the “phase rule” applies to all pure and combinedelements and strictly dictates the distribution of liquid and solidphases which can exist for specific compositions. However, by usingpowder metallurgy considerations of solid-liquid phase changes can beignored, so the process is more flexible than casting, extrusion, orforging techniques. It is therefore possible to fabricate componentswhich otherwise would decompose or disintegrate. In the present case,the stable austenitic phase of the desired composition is narrow (interms of temperature) under conventional processes (welding, casting orP-ESR) and the presently claimed powder metallurgical process allows forthe production of a stable phase without the likely impurities thatwould otherwise arise.

The claimed manufacturing process produces very little scrap metal andallows for the production of different product shapes. The tolerancesthat this process can do are very precise, ranging from +/−0.02 cm foraxial dimensions and +/−0.05 cm for radial dimensions.

The method and composition of the present invention can be used toproduce steel products such as bearing components. The bearing componentwill typically have been formed by a process involving hardening andtempering. As a consequence, the microstructure will generally comprisemartensite, together with any retained austenite and also carbidesand/or carbonitrides. In one embodiment, the bearing component may besubstantially austenitic with just the surface wear portion subjected tomartensitic through-hardening. Alternatively, the whole product may betempered and hardened by conventional methods to produce amartensitically hardened product.

The present invention also provides for a bearing component formed bythe method or made of the steel composition as herein described. Thebearing component may be at least one of a rolling element, an innerring, and an outer ring.

At least a wear portion of the bearing component is formed by the methodor made of the steel composition as herein described. The wear portion,for example, the raceway of a bearing ring, may be formed separatelyfrom the body of the bearing ring and then joined by conventionaljoining techniques such as diffusion joining or welding.

The present invention also provides for a bearing comprising a bearingcomponent as herein described.

Bearings may be used in many different types of machinery to retain andsupport rotating components such as, for example, a wheel on a vehicle,a vane on a windmill or a drum in a washing machine. The presentinvention is particularly suited for the manufacture of bearing ringsfor large-size bearings (LSB). LSBs have an outside diameter of 450 cmor greater.

The bearing of the present invention is particularly suitable for use asa turbine bearing, in particular a wind turbine or wind mill bearing.Under such applications the high corrosion resistance and strengthcharacteristics are key.

In a preferred embodiment, the method of the present invention can beused to adhere a wear portion of a bearing to the body of a bearingcomponent. That is, the body of the bearing component forms a part ofthe mould and the wear portion is clad onto the surface using hotisostatic pressing. Thus, for example, a less expensive material can becoated with a thin layer of powdered steel composition to create asuitable surface for withstanding rolling contact fatigure. This reducescosts by placing expensive, wear resistant materials only where they areneeded. As a result, wear resistant properties are improved withoutincurring unnecessary cost penalties. An additional benefit of claddingis that it can create bonds between otherwise incompatible materialssuch as metal, intermetallic, and ceramic powders.

The hardness of the steel composition as an article is preferably 50 HRCor more, and most preferably 55 HRC or 58 HRC or more. The hardness ismeasured according to the Rockwell hardness test which is well known inthe art. HRC is measured with a minor load of 98 N and a diamond conemajor load of 1372N.

EXAMPLE

With reference to FIGS. 1 and 2 the production of a stainless steelbearing will now be described by way of a non-limiting example.

As is shown in FIG. 1, a steel composition 6 was prepared in aninduction furnace 5. Ingredients 2 necessary for providing the desiredcomposition were fed into the induction furnace 5. Two electrodes 4connected to an alternating power supply, and placed within theinduction furnace 5 were used to heat the ingredients 2 to provide amolten steel composition 6. Optionally, the molten composition 6 couldhave been held in a tundish (not shown) under a protective atmosphereprior to atomisation.

The ingredients 2 were selected to provide the following steelcomposition 6.

TABLE 1 All in wt %. Material Balance C N Cr Mo Si Mn Fe Target Fe 0.460.20 15 1 0.5 0.5 82.34

Once the steel composition 6 was fully molten it was slowly dischargedfrom the induction furnace 5 into an expansion chamber 10. The moltensteel composition 6 was contacted with nitrogen air jets provided from ablower 8. The blown steel composition 6 became a number of finelydispersed lines of steel 12, which became increasingly fragmented withinthe expansion chamber 10 due to turbulence and formed a powder 14. Thepowder 14 was collected in the base of the expansion chamber 10.

In a packing step 16 the powder 14 was loaded into mild steel moulds 18shaped to form the body of a bearing component and lightly compressed toform a weak cohesive structure.

In a hot isostatic pressing step 20 the filed moulds were subjected to atemperature of 1200° C. and a pressure of 95 MPa for 3 hours. Hotisostatic pressing was conducted under an argon atmosphere.

In a finishing treatment step 24 the mild steel moulds 18 were removedby pickling and the bearing component was removed. The bearing wassubstantially all austenite. The bearing component was then subjected toa step of tempering and hardening to form a martensitically hardenedbearing component.

Turning to FIG. 2, the foregoing process can be summarised in the stepsof:

-   -   1. providing the steel composition 6, which was achieved by        melting the ingredients 2 together in the induction furnace 5.    -   2. powdering the steel composition 6, which was achieved by        atomisation with air jets in the expansion chamber 10.    -   3. packing the steel powder 14 into moulds 18.    -   4. hot isostatically pressing 20 the moulds 18.    -   5. finishing the pressed steel article by removing the mould 18.

The bearing component was analysed to confirm its final composition andproperties. The final composition is shown in table 2.

TABLE 2 Chemical composition of the final composition. All in wt %.Material Balance C N Cr Mo Si Mn Fe Composition Fe 0.46 0.20 15 1 0.50.5 82.34

The bearing component was found to exhibit excellent corrosionresistance, strength (tensile strength) and hardness.

Having sufficient interstitial species soluble in austenite duringhardening is clearly important to achieve the hardness level which isrequired in the bearing material. The carbon and nitrogen concentrationswould amount, in the current case, to 0.6 wt %.

In view of the foregoing the present invention enables the production ofbearing components having particularly high corrosion resistance,strength and hardness.

Furthermore, by using a powder metallurgical synthesis route thecomposition avoids defects and unwanted impurity phases, has a finergrain size and exhibits even greater corrosion resistance, strength andhardness.

1. A bearing steel composition comprising: Carbon 0.4 to 0.8 wt %;Nitrogen 0.1 to 0.2 wt %; Chromium 12 to 18 wt %; Molybdenum 0.7 to 1.3wt %; Silicon 0.3 to 1 wt %; Manganese 0.2 to 0.8 wt %; and Iron 78 to86.3 wt %.


2. A bearing steel composition according to claim 1, wherein thecombined carbon and nitrogen content is from 0.5 to 0.7 wt % of thetotal composition.
 3. A bearing steel composition according to claim 1,wherein the steel composition comprises at least one of the elements inan amount of: Carbon 0.45 to 0.7 w %; Nitrogen 0.11 to 0.18 wt %;Chromium 14 to 17 wt %; Molybdenum 0.9 to 1.1 wt °70; Silicon 0.5 to 0.9wt %; and Manganese 0.3 to 0.6 wt %.


4. A bearing steel composition according to claim 1, wherein the steelcomposition comprises: Carbon 0.45 to 0.6 wt %; Nitrogen 0.13 to 0.17 wt%; Chromium 15 to 16.5 wt %; Molybdenum 0.9 to 1.1 wt %; Silicon 0.5 to0.9 wt %; and Manganese 0.3 to 0.6 wt % and

the balance iron and unavoidable impurities.
 5. A bearing steelcomposition according to claim 1 and having a microstructure comprisingmartensite, retained austenite and precipitated carbides and/orcarbonitrides.
 6. A bearing component formed from a compositionaccording to claim
 1. 7. A bearing component according to claim 6 whichis at least one of a rolling element, an inner ring, and an outer ring.8. A bearing comprising a bearing component as claimed in claim
 6. 9. Amethod of forming a bearing component comprising: a) providing apowdered steel composition according to claim 1; and b) subjecting thepowdered steel composition to hot isostatic pressing to form saidcomponent.
 10. A method according to claim 9, wherein the powdered steelcomposition is provided by atomising a molten steel composition bycontacting it with at least one jet of gas.
 11. A method according toclaim 9, wherein the step of hot isostatic pressing is conducted at atemperature of from 1000° C. to 1400° C. and a pressure of up to 200MPa.
 12. A method according to claim 9, wherein the step of hotisostatic pressing is conducted under an inert atmosphere.
 13. A methodaccording to claim 9, further comprising a step of: a) case hardening;b) tempering; and/or c) finishing.